Myopathies are disorders that result in functional impairment of muscles. Muscular dystrophy (MD) refers to genetic diseases that are characterized by progressive weakness and degeneration of skeletal muscles. Duchenne muscular dystrophy (DMD) and Becker muscular dystrophy (BMD) are the most common childhood forms of muscular dystrophy. They are recessive disorders and because the gene responsible for DMD and BMD resides on the X-chromosome, mutations mainly affect males with an incidence of about 1 in 3500 boys.
DMD and BMD are caused by genetic defects in the DMD gene encoding dystrophin, a muscle protein that is required for interactions between the cytoskeleton and the extracellular matrix to maintain muscle fiber stability during contraction. DMD is a severe, lethal neuromuscular disorder resulting in a dependency on wheelchair support before the age of 12 and DMD patients often die before the age of thirty due to respiratory- or heart failure. In contrast, BMD patients often remain ambulatory until later in life, and have near normal life expectancies. DMD mutations in the DMD gene are mainly characterized by frame shifting insertions or deletions or nonsense point mutations, resulting in the absence of functional dystrophin. BMD mutations in general keep the reading frame intact, allowing synthesis of a partly functional dystrophin.
During the last decade, specific modification of splicing in order to restore the disrupted reading frame of the DMD transcript has emerged as a promising therapy for Duchenne muscular dystrophy (DMD) (van Ommen, van Deutekom, Aartsma-Rus, Curr Opin Mol Ther. 2008; 10(2):140-9, Yokota, Duddy, Partidge, Acta Myol. 2007; 26(3):179-84, van Deutekom et al., N Engl J Med. 2007; 357(26):2677-86. Using antisense oligonucleotides (AONs) interfering with splicing signals the skipping of specific exons can be induced in the DMD pre-mRNA, thus restoring the open reading frame and converting the severe DMD into a milder BMD phenotype (van Deutekom et al. Hum Mol Genet. 2001; 10: 1547-54; Aartsma-Rus et al., Hum Mol Genet 2003; 12(8):907-14.). In vivo proof-of-concept was first obtained in the mdx mouse model, which is dystrophin-deficient due to a nonsense mutation in exon 23. Intramuscular and intravenous injections of AONs targeting the mutated exon 23 restored dystrophin expression for at least three months (Lu et al. Nat Med. 2003; 8: 1009-14; Lu et al., Proc Natl Acad Sci USA. 2005; 102(1):198-203). This was accompanied by restoration of dystrophin-associated proteins at the fiber membrane as well as functional improvement of the treated muscle. In vivo skipping of human exons has also been achieved in the hDMD mouse model, which contains a complete copy of the human DMD gene integrated in chromosome 5 of the mouse (Bremmer-Bout et al. Molecular Therapy. 2004; 10: 232-40; 't Hoen et al. J Biol Chem. 2008; 283: 5899-907).
As the majority of DMD patients have deletions that cluster in hotspot regions, the skipping of a small number of exons is applicable to relatively large numbers of patients. The actual applicability of exon skipping can be determined for deletions, duplications and point mutations reported in DMD mutation databases such as the Leiden DMD mutation database available at www.dmd.nl. Therapeutic skipping of exon 45 of the DMD pre-mRNA would restore the open reading frame of DMD patients having deletions including but not limited to exons 12-44, 18-44, 44, 46, 46-47, 46-48, 46-49, 46-51, 46-53, 46-55, 46-59, 46-60 of the DMD pre-mRNA, occurring in a total of 16% of all DMD patients with a deletion (Aartsma-Rus and van Deutekom, 2007, Antisense Elements (Genetics) Research Focus, 2007 Nova Science Publishers, Inc). Furthermore, for some DMD patients the simultaneous skipping of one of more exons in addition to exon 45, such as exons 51 or 53 is required to restore the correct reading frame. None-limiting examples include patients with a deletion of exons 46-50 requiring the co-skipping of exons 45 and 51, or with a deletion of exons 46-52 requiring the co-skipping of exons 45 and 53.
Recently, a first-in-man study was successfully completed where an AON inducing the skipping of exon 51 was injected into a small area of the tibialis anterior muscle of four DMD patients. Novel dystrophin expression was observed in the majority of muscle fibers in all four patients treated, and the AON was safe and well tolerated (van Deutekom et al. N Engl J Med. 2007; 357: 2677-86).
Most AONs studied contain up to 20 nucleotides, and it has been argued that this relatively short size improves the tissue distribution and/or cell penetration of an AON. However, such short AONs will result in a limited specificity due to an increased risk for the presence of identical sequences elsewhere in the genome, and a limited target binding or target affinity due to a low free energy of the AON-target complex. Therefore the inventors decided to design new and optionally improved oligonucleotides that would not exhibit all of these drawbacks.